1. Field of the Invention
The present invention relates to a system for controlling a driving force in a hybrid vehicle which is equipped as its motive force source with an internal combustion engine such as a gasoline engine or a Diesel engine and an electric motor such as a motor or a motor/generator to be activated by an electric power for outputting a torque.
2. Related Art
As well known in the art, the internal combustion engine inevitably emits exhaust gases. The components and amounts of the exhaust gases are dependent upon the running state of the internal combustion engine. At a high load running time when the throttle opening is increased at a low vehicle speed, according to a general tendency, it is liable to lower the cleanness of the exhaust gases and the fuel economy. In recent years, on the other hand, a demand for the cleanness of the exhaust gases of the vehicle having the internal combustion engine mounted thereon has grown higher and higher. In order to satisfy this demand, there has been developed a hybrid vehicle which is equipped with the internal combustion engine and an electric motor as its motive force source. In the hybrid vehicle of this kind, the motive force source can be selected according to the running state or the demanding state of the driving force so that the internal combustion engine can be run in the most efficient state. As a result, the fuel economy can be improved better than the prior art, and the emission of the so-called xe2x80x9cgreenhouse gasesxe2x80x9d can be reduced.
On the other hand, there is known as the type of the hybrid vehicle the so-called xe2x80x9cparallel hybridxe2x80x9d, in which an electric motor and a power generator are coupled to a power transmission line such as a transmission or a drive shaft and in which the internal combustion engine is selectively coupled to the power transmission line by a coupling mechanism such as a clutch so that the output of the internal combustion engine may be used as the motive powers for generating the electric power and for driving the hybrid vehicle. At a low vehicle speed time when the internal combustion engine drops its working efficiency or comes into an unstable running state, this parallel hybrid vehicle is driven to run by the electric motor, and the internal combustion engine is started when the vehicle speed rises to some level. In this case, the torque of the electric motor can be transmitted to the internal combustion engine through application means such as the clutch so that the internal combustion engine can be rotated and started if the application means is applied while the vehicle is driven to run by the electric motor. With this construction, the starter, as has been employed in the general vehicle of the prior art, can be done away with to reduce the size and weight of the drive unit.
When the internal combustion engine is not started before the vehicle speed rises to some extent, as described above, the coupling mechanism can be released at a low vehicle speed time or at a starting time to uncouple the internal combustion engine from the power transmission line so that the internal combustion engine can be left at a stop. A system for this control has been disclosed in Japanese Patent Laid-Open No. 37411/1997. According to this system, the output of the electric motor is not wasted even partially by the friction, as accompanying the mere rotation of the internal combustion engine, so that the power loss can be prevented to improve the energy efficiency.
Here, the internal combustion engine such as a gasoline engine or a Diesel engine can be fed with a fuel and ignited even in a state having a speed lower than the idle level. In this state of a low speed, however, the combustion is not stabilized to make the output torque or output rotation unstable. In the system, as disclosed in the aforementioned Laid-Open, the internal combustion engine is uncoupled at a low vehicle speed time from the power transmission line and is left at a stop. In addition to the aforementioned prevention of the power loss, therefore, it is possible to prevent the fluctuation in the driving torque and the resultant deterioration in the riding comfort.
However, the system of the prior art thus far described is constructed such that the internal combustion engine is completely stopped at the low vehicle speed time. When the vehicle is to be accelerated from the low speed running state, therefore, the vehicle is driven by the output of the electric motor till the speed of the electric motor rises to the level corresponding to the idle speed of the internal combustion engine, and the internal combustion engine is then coupled to the power transmission line and fed with the fuel so that it may be started. This makes it impossible for the internal combustion engine to output a high torque before the speed of the electric motor rises to the level corresponding to the idle speed of the internal combustion engine. As a result, it takes a long time to generate a driving force sufficient for a demand for an acceleration after the driver does an operation for the acceleration. This raises a disadvantage that the vehicle is deficient in a responsibility to the acceleration.
When the vehicle is to be started with a high running resistance as on a steep uphill, on the other hand, the vehicle speed cannot be raised if the output torque of the electric motor is relatively short. Since the internal combustion engine cannot be started in this case, the drive by the electric motor has to be continued but may be unable to satisfy the demand for the, acceleration or may lower the charging amount of a battery abnormally.
According to the system of the prior art, moreover, when the vehicle is decelerated for a temporary stop, the internal combustion engine is uncoupled from the power transmission line to make the stop. When the vehicle is accelerated again after the temporary stop, however, the internal combustion engine cannot be started till the speed of the electric motor reaches the level corresponding to the idle speed of the internal combustion engine. This causes disadvantages that the vehicle is deficient in the acceleration and its responsibility.
An object of the invention is to provide a drive control system for starting an internal combustion engine easily even at a low vehicle speed time just after the start of a hybrid vehicle which is equipped with the internal combustion engine and an electric motor as its motive force source.
Another object of the invention is to prevent the shock which might otherwise be caused at the time of starting the internal combustion engine or coupling the started internal combustion engine to the power transmission line.
According to the invention, therefore, there is provided motoring means for coupling the internal combustion engine to the power transmission line to rotate the internal combustion engine, with the feed of a fuel to the internal combustion engine being stopped when the hybrid vehicle is driven to run by the output of the electric motor, by controlling a clutch mechanism in an applied state. As a result, the internal combustion engine is dragged, as it is called so, while the vehicle is driven to run by the output of the electric motor. When the internal combustion engine has to be started in response to a demand for raising the driving force or for generating the electric power, therefore, it can be instantly started by feeding it with the fuel. As a result, a delay in the response to such demand can be avoided.
In the invention, moreover, the internal combustion engine and the power transmission line can be uncoupled by controlling the clutch mechanism in a released state when the vehicle speed or the input speed of a transmission is at a predetermined or higher level or when the gear ratio is at a medium or low level below a predetermined level when the hybrid vehicle is driven to run by the output of the electric motor. With this construction, while the vehicle is driven to run by the output of the electric motor, on the other hand, the internal combustion engine is rotated by coupling it to the power transmission line only either when the vehicle speed or the input speed of the transmission is low or when the gear ratio is high. By starting the feed of the fuel to the internal combustion engine while the vehicle is running with the electric motor, the internal combustion engine can be started so that the shock to be caused by the start of the internal combustion engine can be easily reduced. Since the speed of the internal combustion engine having no feed of the fuel is low, moreover, the power loss can be suppressed. In the running state contrary to this, that is, while the vehicle is running with the power of the electric motor and when the vehicle speed or the input speed of the transmission is high or when the gear ratio is low, the internal combustion engine is uncoupled from the power transmission line and is stopped so that the power loss, as might otherwise accompany the rotation of the internal combustion engine, will not occur. Even when the internal combustion engine is rotated to be started, moreover, the change in the driving torque can be reduced to prevent the deterioration in the shock.
In the invention, moreover, while being rotated by the output of the electric motor, the speed of the internal combustion engine can be limited to a predetermined level.
With this construction, the speed of the internal combustion engine having no feed of the fuel can be restricted to a low level so that the power loss due to the friction in the internal combustion engine or the compression of the air can be suppressed. If the speed of the internal combustion engine is restricted by restricting the torque transmission capacity of a coupling mechanism, moreover, the torque fluctuation accompanying the rotation of the internal combustion engine can be suppressed from appearing in the driving torque.
When the vehicle is to be started by the output of the electric motor, according to the invention, the internal combustion engine is coupled to the power transmission line by the clutch mechanism so that it may be rotated, and the internal combustion engine can be started by feeding it with the fuel at a predetermined speed and then uncoupled from the power transmission line. With this construction, since the internal combustion engine is, rotated by the torque of the electric motor from the start by the output of the electric motor, it can be started by feeding it with the fuel even when the speed of the electric motor or the vehicle speed is low. If the combustion in the internal combustion engine is continuous, on the other hand, the internal combustion engine is uncoupled from the power transmission line. As a result, even if the combustion or the output torque is unstable because the speed of the internal combustion engine is low, it is possible to prevent the fluctuation in the driving torque.
Here in the invention, the internal combustion engine can be rotated and started by the motive power of the electric motor after it was started by the output of the electric motor, and then can be uncoupled from the power transmission line. With this construction, the time period for driving the internal combustion engine with the electric motor is shortened to suppress the power loss which might otherwise be caused by rotating the internal combustion engine with no feed of the fuel.
The system of the invention can be also constructed such that the time point when the combustion of the fuel is started by igniting the internal combustion engine, i.e., when the internal combustion engine is substantially started can be decided on the basis of the speed of the internal combustion engine to be driven by the electric motor. With this construction, too, it can be easily started at the low speed of the vehicle or the electric motor, and the fluctuation in the output torque of the internal combustion engine in the case where the engine is started at the low speed does not appear in the driving torque so that the deterioration in the riding comfort can be prevented.
In the invention, moreover, there can be further provided application control means for applying again the released clutch mechanism so that the speed of the internal combustion engine may reach a predetermined level. With this construction, the speeds of the electric motor and the internal combustion engine are synchronized at the time point when the clutch mechanism is applied, so that the run of the vehicle can be changed from that by the electric motor to either that by the internal combustion engine or that by the electric motor and the internal combustion engine.
On the other hand, the invention can be provided with a mechanism for coupling the internal combustion engine to the power transmission line through a torque synthesizing/distributing mechanism capable of performing a differential action and for integrating the torque synthesizing/distributing mechanism. Since the transmission of the torque between the internal combustion engine and the power transmission line can be controlled by the torque synthesizing/distributing mechanism, the internal combustion engine can be started as in the foregoing individual constructions even at a starting time or at a low vehicle speed time by the electric motor, and the power loss, as might otherwise be caused by rotating the internal combustion engine unnecessarily by the electric motor, can be reduced. Since the torque synthesizing/distributing mechanism performs the differential action, moreover, the speed of the internal combustion engine can be controlled even while the vehicle is at a stop. This makes it possible to start and accelerate the vehicle easily from the stop time.
The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read with reference to the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and are not intended as a definition of the limits of the invention.